An electrical appliance or load receives electrical energy from one terminal of an electrical outlet or source (the so-called high voltage or hot side), electrical current flows to the load through an electrical conductor (a wire), this current passes through the load and is then returned to another terminal of the electrical outlet through another wire called the neutral wire. The two wires that connect source and load may have a coating of rubber or some other electrical insulating material or they may be bare, in which case air, which is a good insulator, functions to inhibit electrical current flow outside of the wire. Since the human body can conduct the flow of electrical current, if a person comes into contact with one electrified object, such as the so-called hot wire in an electrical system, while also making contact with a second object having a substantially different voltage, then an electrical leakage current that is proportional to the voltage difference will flow through the person and may cause injury or death. If the second object that the person comes in contact with is electrically connected to the earth (“ground”) then this is called a ground fault.
Electrical current is the flow of electrons. Electrons are neither created nor destroyed so any functioning electrical appliance will require both an entry path for electrons and an exit path for electrons in order for electrical current to flow. For most household appliances that operate from a plug, electrons will sometimes enter path one and exit path two and sometimes enter path two and exit path one. This is known as alternating current or AC operation.
Although the two conductors coming out of an AC power source are often designated as “hot” and “neutral”, in an AC system, the hot conductor will cyclically have a more positive voltage than the neutral for half the time and will cyclically have a more negative voltage than neutral for half the time, having a momentary value of zero (the so-called “zero crossing”) each time the voltage passes from positive to negative or from negative to positive.
Any electrical appliance that plugs into a wall outlet can be the source of electrical shock, electrical burns, and possibly electrocution. Worn or damaged appliances or appliance cords may come in contact with a conducting surface that is accessible to the user, thus presenting a hazardous condition. For example, if the rubber insulation on a wire within an appliance is worn, then the exposed metal wire strands might touch the appliance housing. If the appliance housing were made out of metal or a similar electrical conducting material then a shock hazard could exist.
In order to minimize the potential hazards intrinsic to electrical appliances, government and industry standards serve to regulate the way in which appliances are built and used. One outcome is that many classes of electrical appliances are required to have a grounded connection over and above the standard two electrically conductive wires that supply electrical power to the appliance. A grounded electrical cord is recognizable because it has a plug with three prongs. In the U.S., in a 120 volt (sometime referred to nominally as 110 volts, 115 volts, or 120 volts) single phase system, two of the prongs on a grounded plug are flat and the third, ground prong, is generally rounded.
To achieve electrical protection, the grounding line will connect to ground at the plug where it is plugged into a grounded outlet. The other end of the grounding line, within the cordset, will connect to the appliance housing or other exposed or potentially exposable metal parts. If an electrically hot (that is, having a voltage potential that is significantly different from a ground potential) conductor comes into contact with a grounded conductor, the grounded conductor will present a low resistance path to ground, causing the hot conductor to maintain a voltage at or near a ground potential. If the current drawn from an electrically hot source, through ground, exceeds the “trip” current of an in-line circuit breaker, the circuit breaker will open, removing power from the outlet that the appliance is plugged into, thereby protecting the electrical distribution system from over-current damage and protecting the user from electrical shocks.
If the ground connection on a grounded appliance is removed, the safety of the appliance is compromised. This removal of ground condition can occur through abuse or damage but more commonly occurs when the appliance is plugged into an outlet in which the ground has not been connected or is improperly wired. The removal of ground may also occur if the grounding prong is cut off of the plug or is otherwise disconnected. An open ground condition often goes unnoticed by the casual user. This is because most appliances will operate normally without a ground connection.
In the U.S., agencies such as the National Fire Protection Association (NFPA) and the Occupational Safety and Health Association (OSHA) maintain regulations governing the use of grounded appliances. For many situations, the regulations require that a qualified technician periodically test the continuity of the ground connection between appliance and the grounding prong and between grounding prong and actual ground.
One drawback to the periodic test approach is that a significant period of time may elapse between inspections. In many situations the inspection schedule may not be adhered to or may be completely ignored. Also, malfunctions, particularly if intermittent, may go undetected. Often a complete inspection of the grounding impedance between actual ground is not made. Accordingly, there are many situations where a user may assume he is using a grounded appliance when he is not.
Another potentially hazardous operating condition occurs when the hot and the neutral conductors are switched at an outlet. The typical appliance that is plugged into an outlet with these so-called “transposed” conductors will still operate. However, the design of many appliances is such as to favor having one of the two power carrying conductors designated as the neutral conductor. For example, in a droplight, the socket for an incandescent light bulb will have the outlet shell as neutral with the base connected to the hot conductor. Then if a user accidentally makes contact with the exposed outer shell, electrical shock is less likely to result. The neutral and hot conductors are distinguished by using a polarized plug wherein one of the two blades is designated as neutral and is physically wider. If a droplight is plugged into a polarized outlet but the hot and neutral conductors in that outlet are transposed, the safety associated with using a polarized plug has been compromised. In such situations, it is important to alert the user.
In the prior art, U.S. Pat. No. 3,697,808 (Lee) discloses a system for monitoring chassis potential and ground continuity by injecting a radio frequency (RF) signal into the neutral lead and monitoring the signal amount that is capacitively coupled to ground. A major limitation of this approach is that it is electronically complicated and is sensitive to leakages through capacitances in the power cable and elsewhere.
U.S. Pat. No. 3,809,961 (Kershaw) discloses an electrical outlet sentinel that detects an open ground condition in an electrical outlet and opens a mechanical circuit breaker thereby removing power from the outlet. A major objection to this design is that power from the outlet is controlled by a circuit breaker whose relay is energized through power running continuously through ground. For any practical relay, this represents a substantial continuous ground current and is likely to disrupt the correct operation of any ground fault circuit interrupters that are upstream in the electrical distribution system.
U.S. Pat. No. 3,996,496 (Volk) discloses a ground integrity monitor that relies on the application of short electrical pulses between the neutral and ground terminals. If the resistance between neutral and ground is less than a threshold amount, this pulsing purportedly causes a photo coupler to be activated, providing power to the load. The system is electronically complicated and this compromises the appeal of the design.
U.S. Pat. No. 4,598,331 (Legatti) discloses a ground fault interrupt circuit in which open ground conditions create actuation of the interrupter. An open ground produces current flow through a supplemental secondary winding that, in turn, induces a trip signal. When implemented, this approach does not check for a good connection going forward to the appliance, but only going back to the outlet. If, for example, the grounding conductor on the appliance cordset is cut, thereby leaving the appliance ungrounded, this approach will not detect that condition.
U.S. Pat. No. 4,649,454 (Winterton, et al.) discloses an open ground detection circuit for appliances that utilizes a special plug with four electrical connections. Two of these are the standard hot and neutral prongs. A third prong, having the location and shape of a conventional ground prong, has two electrical conductors (the so-called grounding blade and sensing blade) that are separated by an insulating member. Four wires connect this special plug to a housing that is located within an appliance. When the special plug is correctly seated in a three-hole grounded outlet, the two blades on the third prong are electrically connected together. If, however, the two conductors on the third prong are not electrically connected together, then an alarm signals that an open ground condition exists. A major problem with this design is that it only checks to see whether grounding blade and sensing blade are shorted together. This can occur even if the plug is connected to an ungrounded outlet and the user would incorrectly assume that the appliance was grounded. Another problem with this design is that it requires a special plug and four wires to connect between the appliance and this special plug.
U.S. Pat. No. 4,931,893 (Glennon) discloses a circuit that detects a loss of ground condition in an electrical system by using a capacitive circuit that discharges through the ground. The disclosed embodiments are electronically complex and can generate relatively high ground currents which would cause nuisance tripping in distribution systems having ground fault interrupters.
U.S. Pat. Nos. 5,844,795 and 5,943,198 and U.S. patent application Ser. No. 08/756,784, all to Hirsch et al., describe a solid state ground and arc fault detection and interruption technology that has two parts, one part which resides in the load and one part which resides in the source. In applying this to an electrical appliance, the load is the appliance and the source is the plug. The basic theory in this technology is that a load conditioning module in the appliance injects a deadzone in the current flow during each half wave AC cycle. A sensing circuit in the plug looks for the presence of that deadzone each half cycle. If there is leakage around the load conditioning module (indicating a ground fault or arcing fault) this is indicative of a potential dangerous condition and current flow is interrupted at the plug.
To address these representative deficiencies in the art, what is needed is an improved capability for controlling electrical power to an appliance in a manner that provides safety to people that may come in contact with the appliance.